Determination of preservice science teachers' conceptions and alternative conceptions about ionization energy were aimed in this study. To achieve this aim, a two-tier multiple-choice test with ten questions was implemented to 300 preservice science teachers in their second and third year in Science Division of Fatih Faculty of Education at Karadeniz Technical University. The test was taken from literature. Researcher redesigned the original form of the test for the sample and the phrase "I do not know the answer" found in the first section of the original questions was extracted from the question for this study. Both first and second parts of the questions were analyzed and the percentages of students' responses were calculated. Results showed that most of the preservice science teachers did not have an adequate understanding on ionization energy and had different alternative conceptions. (C) 2010 Published by Elsevier Ltd.